Multiple behavioral mechanisms shape social behavioral development in a highly social cichlid fish

Early-life social experiences shape adult phenotype, yet the underlying behavioural mechanisms remain poorly understood. We manipulated early-life social experience in the highly social African cichlid fish Astatotilapia burtoni to investigate the effects on behaviour and neuroendocrine stress axis function. Juveniles experienced different numbers of early-life social partners in stable pairs (1 partner), stable groups (6 fish; 5 partners), and socialized pairs (a novel fish was exchanged every 5 days; 5 partners). Treatments differed in group size (groups vs. pairs) and stability (stable vs. socialized). We then measured behaviour in multiple contexts and collected water-borne cortisol. We found effects of treatment on behaviour across all assays: open field exploration, social cue investigation, dominant behaviour, and subordinate behaviour. Cortisol did not differ across treatments. Principal components (PC) analysis revealed robust co- variation of behaviour across contexts, including with cortisol, to form behavioural syndromes sensitive to early-life social experience. PC1 (25.1%) differed by numbers of social partners: juveniles with more social partners were more active during the social cue investigation, spent less time in the territory, and were more interactive as dominants. Differences in PC5 (8.5%) were based on stability: socialized pairs were more dominant, spent less time in and around the territory, were more socially investigative, and had lower cortisol than stable groups or pairs. Behaviour observations in the home tanks provided further insights into the behavioural mechanisms underlying these effects. These results contribute to our understanding of how early- life social experiences are accrued and exert strong, lasting effects on adult phenotype.